1. Field of the Invention
This invention relates to an automatic camera focus detecting device for automatically detecting the camera focus by making use of the changing contrast of the image of the object.
2. Description of Prior Art
The optical image of an object focused by the photographing lens has a characteristic such that the brightness and darkness of the image, i.e., the contrast thereof, is maximum when the image is precisely focused. This phenomenon can be explained by the fact that the light intensity (power spectrum) of the image of the object at each spacial frequency is maximum when the image is precisely focused. Thus, the focus can be automatically detected from the detection of the maximum contrast.
The hitherto proposed methods for extracting the contrast information itself include one method which uses nonlinear photoconductive elements such as CdS, and one difference between the photoelectric outputs of a pair of microphotoelectric elements adjacent to each other selected from a number of such elements arranged on the image forming plane is statically detected, or another method which uses a self-scanning type photoelectric converting device including a plurality of microphotoelectric elements arranged in a single row and a scanning circuit for the microphotoelectric element group for dynamically obtaining a contrast signal from the photoelectric outputs of the microphotoelectric elements, as disclosed in Japanese Utility Model Publication No. 95830/78 corresponding to U.S. Pat. No. 3,896,304, Japanese Patent Publication No. 29878/1980 corresponding to U.S. Pat. No. 4,255,028 and Japanese Patent Publication No. 35317/1980 corresponding to U.S. Pat. No. 4,298,254 by the same applicant as the instant application.
The state of focus can be detected by using the above contrast detecting elements in various methods. In one example of the method, the maximum value state of a contrast signal is detected with a single contrast detecting element disposed at a position equivalent to the image plane. In another example, a single contrast detecting element is mechanically oscillated on the optical axis back and forth in a plane equivalent to the image plane for analyzing the changes of the contrast output in the oscillation period, as disclosed in, for instance, Japanese Patent Publication No. 29832/1980 corresponding to U.S. Pat. No. 4,264,160 and Japanese Utility Model Publication No. 95830/1978 Corresponding to U.S. Pat. No. 3,896,304. In a further example, two contrast detecting elements in respective positions before and after and at an equal distance from a position which is equivalent to the image plane, and the state in which the contrast signal outputs from both these contrast detecting elements are equal is detected, as disclosed in, for instance, Japanese Patent Application No. 15257/1979.
These automatic focus detecting systems which make use of the contrast information of the image of the object are suited for assembly as a commonly termed TTL focus detecting device in an automatic focus detecting device of a single-lens reflex camera.
Meanwhile, the single-lens reflex camera or like high class camera, which is capable of replacement of lens, covers a wide range of the photographing distance. Also, since it uses a high performance lens, a sharp image can be obtained, and the brightness of the object that can be photographed is wide.
Therefore, the automatic focus detecting device has to meet various stringent conditions if it is to be used for the single-lens reflex camera as mentioned. More particularly, it is required to cover a wide range of the brightness of the object, to have high precision of focus, to be capable of application even where a swing is present or the object is being moved, and to be applicable for use various lenses that can be replaced with one another. However, these conditions have not always been satisfactorily met by the prior art devices or systems as mentioned.
Where the aforementioned two contrast detecting elements are arranged in their positions before and after a position equivalent to the image plane, frequent use has been made of a system of splitting the light path into two paths with a light path divider including a half reflection mirror, a total reflection mirror, etc. in order to provide a small-size and compact construction.
However, where such a light path divider is used, an unbalanced construction results from the provision of only a half reflection mirror in one of the two branch light paths and a half reflection mirror and a total reflection mirror in the other branch light path, that is, the two branch light paths have different light transmittances (i.e., ratios of the quantity of output light to the quantity of incident light). Therefore, despite the fact that two contrast detecting elements are used, the accuracy of the focus detection has not been sufficiently high.